1. Field of the Invention
The present invention is broadly concerned with an improved method and apparatus for the rapid, accurate determination of the water-cement ratios of fresh, uncured concrete. More particularly, the invention pertains to a method and apparatus wherein a dilute dispersion or paste of the cement fraction of the concrete is passed through a turbidimeter to determine the turbidity of the dispersion; this value can then be compared with a known, standard turbidity curve to ascertain the water-cement ratio of the fresh concrete.
2. Description of the Prior Art
Concrete is a man-made composite, usually made up of cement, aggregate and water; various types and sizes of aggregates can be used, together with additives such as air-entraining agents. Concrete is one of the most widely used of construction materials. However, concrete is unique in that it is manufactured as used and generally cannot be tested for acceptance in advance. Such acceptance is sometimes based on a strength test at an advanced stage of curing, as concrete is known to gain strength over long periods of time. Acceptance may also be based on adherence to a specified formula, along with monitoring for strength. As a consequence, concrete may be placed which may or may not be acceptable at a later date. If found unacceptable, then the emplaced concrete must be removed and new concrete poured.
Accordingly, there is a real and unsatisfied--need in the art for a means of assessing the quality of concrete before placement, and for assuring that the concrete meets the specified standards. In this regard, the water-cement ratio of a plastic, uncured concrete is known to be a good candidate for assessing the quality of concrete delivered to a construction site. This is because in engineering practice, the strength and permeability of Portland cement concrete of a given age and cured at a prescribed temperature condition is assumed to depend primarily on two factors: the water-cement ratio of the fresh concrete and the degree of compaction. Thus, when full compaction is achieved, the only variable affecting the strength and permeability of any concrete mix is the water-cement ratio. For a fully compacted concrete, this strength is taken to be inversely proportional to the water-cement ratio, according to a relationship established by Duff Abrams in 1919: EQU strength, f.sub.c =K.sub.1 /K.sub.2.sup.w/c
where w/c represents the water-cement ratio of the mix, and K.sub.1 and K.sub.2 are empirical constants.
Although the initial water-cement ratio is the predominant factor affecting strength of a hardened concrete, there is currently no reliable method available for measuring water-cement ratios of fresh concretes in the field or at a job site. An indirect evaluation of water-cement ratio can be made through consistency or workability evaluations using a "slump" test. Although this type of test is rapid and easy to employ, it often gives very inaccurate water-cement ratio results.
Accordingly, there is an unresolved need in the art for an improved method and apparatus for the determination of water-cement ratios in concrete mixtures, especially in the field or at job sites.